horrible performance, fps drops, freezes, malfunctioning vsync

win7 64bit
i5 4590
XFX R7850 Black Edition
Samsung 840 SSD
8gb RAM

all drivers are up to date.


forcing vsync via catalyst control center has no effect, while activating vsync ingame makes for a very unpleasant experience because the fps wouldn't stay locked at 60 but spike to 100+ or even 200+ fps all the time.

the overall performance is very, very poor. when I'm on my own the game is playable, maintaining constant fps is impossible though. the very moment I'm in combat with half a dozen enemies the fps are dropping to 40 or even lower.
in party (2+ people) the game becomes completely unplayable with fps drops below 10 when combat becomes intense. I have the feeling though that different spells affect my framerate differently, because it's not always this bad.
according to MSI afterburner my GPU usage stays below 45% all the time.

and very nasty for a hardcore player: opening strong boxes makes the game freeze for 1-2secs.


for comparison: in diablo3 I am able to maintain 50-60fps on max settings pretty much all the time.


what's going on with my game? :(


*update*
correction: playing solo is a pain as well in a lot of places.
http://i.imgur.com/orbLzvC.jpg

as soon as there are some spells on screen the fps drop like crazy.

Deactivate Vsync.

I always had problems with this on.
My Framerate was ~ 20 fps. Sometimes 35, sometimes 5.

And I have a GTX 690 (its as strong as a GTX 980!!)

I deactivated Vsync and now my FPS are between 150 fps and 250 fps.